A building has several different water flows. One is cold water to end use faucets, a second is cold water to the hot water heater, a third is hot water to end use faucets, and a fourth is used drainwater. The result is complex flow times, flow rates, flow volumes and flow temperatures.
To effectively recover heat from drainwater, one must be able to cool drainwater whenever it flows regardless of the other aforementioned water flows.
Water heaters are well known to consume vast amounts of energy to heat cold water to make it hot for human use in washing and cleaning, and for industrial processes. The resulting hot drainwater (also referred to as wastewater) flows freely to the sewer taking with it all of that heat energy. Generation of energy to heat water releases pollutants including those that cause global warming.
Although it would seem obvious to use heat in drainwater to heat new cold water, thereby reducing energy use and saving money, this seemingly simple heat transfer idea has resisted successful solution in spite of many inventors having tried over a very long time.
It is, therefore, the objective of the present invention to provide a heat exchanger apparatus to remove heat from flowing drainwater, to store that heat within that apparatus, and to limit heat loss of that stored heat to cold drainwater that may flow thereafter.
Another objective is to cool drinking water using cold drainwater.
By way of review, in a building, cold water, always under pressure, flows into the water heater when a hot water faucet is opened. It is that cold water flow that forces the hot water out of that faucet. So when hot water flows, an equal amount of cold water flows into the water heater. In a shower, drainwater is also flowing so all three waters are flowing concurrently.
Drainwater, however, can also flow independent of the other two, as, for example, when a washing machine or a bath drains. The water flows in a building are thus unpredictable as to volume, rate of flow, and temperature.
To recover heat from all drainwater flows, one needs a heat storage medium such as an insulated reservoir of water. Drainwater heat that is recovered and stored must then be able to be transferred into the cold water feeding the water heater whenever it happens to flow.
Drainwater heat recovery involves heating cold water with heat from drainwater. The cold water may feed a water heater thereby saving energy and money.
U.S. Pat. No. 4,619,311, to Vasile, describes a drainwater heat recovery system comprising a copper drainpipe heat exchanger whose exterior is wrapped with a copper coil for cold water to be preheated. This type of tube-on-tube heat exchanger has been long-available, such as that sold by the Solar Research in Brighton, Mich. 48116, as part number 5832. Since the two heat exchangers are in direct physical contact, beneficial heat transfer occurs when both drainwater and cold water are flowing simultaneously as when showering.
However when appliances such as a washing machines or sinks are used, the time delay between filling and draining means that the cold water and the drainwater flow at different times and so there are no useful energy savings. Further, cold drainwater will cool the water in the outer coil. These drawbacks severely limit energy savings.
Further, U.S. Pat. No. 4,619,311, to Vasile is not recommended for horizontal drainpipes found in a great many buildings because the design requires a generally circular drainpipe with no finning upon which to wind the outer coil. This restricts heat transfer to the narrow bottom section of the horizontal drainpipe.
U.S. Pat. No. 5,736,059 to the present applicant, does teach of a drainwater heat recovery system with no-loss heat storage. However, for low volume hot water users, such as in homes, the system tends to be too large and, with its numerous components, too expensive. Further, its installation is essentially limited to vertical drainpipes unless mechanical pumping is added.
The object of the present invention is to provide a low-cost, easy to install drainwater heat exchanger with no-loss heat storage built-in.
A review of the physical principles involved in the present invention follows.
Firstly, when water is heated or cooled, its density changes. Convective flow therefore takes place between colder (heavier) and hotter (lighter) regions.
Secondly, in a vertical pipe or tube, liquid flow is principally a film flow on the wall with no flow down the hollow center.
Thirdly, by adding protrusions to a wall, heat transfer can be improved due to the turbulent flow created.
To achieve one-way heat transfer out of a finned copper drainpipe, in either a vertical or horizontal placement, the present invention uses one or more convection chambers to surround the entire heat transfer surface of the drainpipe. The surrounded drainpipe is submerged in a reservoir of water and a cold water coil is wound around the reservoir. The convection chamber(s) are made of insulative material and are sealed to the drainpipe except for an opening at the top into the reservoir. They fill with a small volume of the reservoir water. In operation, when drainwater flows it""s temperature affects only the convection chamber water. If heated, it becomes lighter and naturally flows by convection out the top into the reservoir heating the reservoir water and thus the cold water coil. If cooled, it becomes heavier and remains stationary in the chamber (no convection) thereby blanketing the drainpipe and preventing warmer reservoir water from being cooled.
By this means, the reservoir is heated whenever hot drainwater flows, and, the the reservoir is cooled whenever the cold water flows. Simultaneous flow of drainwater and cold water is not required and so continuous drainwater heat recovery from both showers and fill-drain devices such as washing machines, baths and sinks.
In the vertical embodiment, multiple convection chambers are used and in the form of nested cups which are open on top and with a hole in the bottom to slide tightly onto the drainpipe. Finning rings are slid down into the cup before the next cup is installed.
In the horizontal embodiment a single tubular convection chamber with each end sealed to the drainpipe and with dividers creating compartments. A slot along the top of the chamber provides the convective opening for reservoir water to flow into and out of the compartments.
In both embodiments, the convection chamber hold a minimum volume of reservoir water but with sufficient space to allow convection flow in reaction to drainpipe temperature.
Convection chambers may be fabricated from low-cost foamed polyethylene or polypropylene. They may also be made of copper and covered with insulation. For the horizontal embodiment, the convection chamber may be a long, channel-shaped trough which may advantageously be made of metal such as copper and attached directly (i.e., soldered) to the bottom of the copper drainwater to enhance heat transfer. Several such metal channels may be nested to further enhance heat transfer. The outside of the outermost convection chamber channel is covered with an insulating skin.
Finning may be made from coiled copper wire or sheet metal.
The reservoir may be a xe2x80x98bagxe2x80x99 made from plastic film supported by the cold water coil encircling it""s exterior. The bag may have a full diameter greater that the external coil to allow bulging of the bag between the outer coil rings thereby adding valuable heat transfer area at no cost.
The larger diameter the plastic tube the better because there will be more volume of water ready for instant delivery to the water heater. Although low-cost plastic conducts heat slowly than expensive copper, the total amount of heat transferred can be the same given the long periods of time involved in real water use in a building.
Multiple drainpipes can be used in one reservoir to maximize heat transfer. Multiple complete units can be connected in series or in parallel where space is a problem.
To increase heat transfer rates, the drainpipe can be pebbled to induce turbulent flow on the interior wall. The drainpipe may also be roll-grooved angularly to slow down flow in horizontal units.
Any shape of drainpipe can be used such as oval.
Miniature drainwater heat recovery systems based on the above described designs may be integrated into the cabinetry of sinks or dish- and laundry washing machines and used with tankless or instantaneous water heaters.
Because indoor plumbing fixtures are heated by ambient air and so even cold water used at the fixture is warmed as it flows over warm fixture and drainpipe surfaces, the system can provide fresh warm water with no need for a traditional hot water supply.
The reservoir of the present invention could be pressurized with fresh water, thus avoiding the cost for the second cold water heat exchanger. This heated water could then be used as feed water for toilets where the heated water would reduce condensation on the exterior of the tank and resultant dripping onto the floor. This dripping is known to cause structural damage, and to support fungus/mold growth which results in dangerous airborne spores in the building.
In yet another embodiment, a thermostatically controlled low wattage heater may be provided within the reservoir to maintain a minimum temperature for use at a site.
Hot drainwater from other sources such as summer roof run off may also be used to heat the cold water.
If the convection chambers are inverted with the convection opening at the bottom, the present invention can serve as a water cooler using colder drainwater to cool warmer fresh water. This arrangement may be practical in hot climates where it is difficult to get cold drinking water in the summer due to high ground temperatures. Such warm xe2x80x98coldxe2x80x99 water when fed into a building can become cooler by evaporative cooling of the fixtures such as standing toilet water. When flushed this colder water can be used to cool the reservoir and the surrounding cold water coil. Small versions of such drainwater cooling units may be used in typical office water coolers to optimize their performance.
Both drainwater heat recovery and drainwater cooling units can be used together to achieve both heating of the cold water being plumbed to the water heater and cooling of the water being plumbed to drinking faucets.